The present invention is directed to a method of gasification and a gasifier. More specifically, the present invention relates to a method of gasification and a gasifier involving cyclonic gasification.
Generally, operation of known cyclonic reactors can present drawbacks. Due to temperature gradients within a cyclonic reactor, there is a tendency for slag to solidify within the reactor, most particularly in the region near where the slag exits the reactor. For example, in known cyclonic reactors, the slag travels through the slag tap and the slag transfers heat by radiation to a cooler environment such as a quench tank. Heat loss from the slag near the slag tap may be relatively high due to the large thermal gradient between the reactor and the quench tank. High heat loss sharply increases the viscosity of the slag, thereby decreasing the flow rate of the slag and often leads to solidification of the slag. This process of slag cooling, viscosity increase, and solidification can lead to a decrease in thermal efficiency for the reactor, an increase in particulate emissions, and/or operational shutdown.
Known cyclonic reactors may erode walls of the reactor by particle-laden flows having high velocity (for example, velocity in excess of about 200 ft/s). In general, when reactor walls include refractory material as a wall insulating material, eroded portions of the refractory material must be replaced regularly to avoid vessel damage or destruction. The replacement of the portions of the refractor wall results in material costs for the replacement material, operational costs for handling the replacement of the refractory material, and an inability to use the reactor during the replacement of the refractory material.
The effectiveness of certain processes and the range of chemical interaction capable is limited by the volume of the reactor. In general, cyclonic reactors involve high velocity injection and also employ relatively high ratios of heat release per unit of volume (for example, in excess of about 10 MW thermal/m3). In order for solid fuels to burn, the solid fuels must first undergo heating, followed by volatilization, then oxidation. Each process is time-dependent and the volume of the reactor affects the duration of time for the process (i.e., for a given heat release, a larger volume permits a longer duration for the process). The known reactors are constrained by the relatively short gas residence time (for example, about one second) available in the cyclonic reactor. Thus, slow burning fuel feedstocks, such as those with high moisture level (for example, exceeding about 15% by weight) or large particle size (for example, having a dimension of about ¼ inch), may not be oxidized to a desirable degree, resulting in reduced fuel utilization and/or reduced efficiency for combustion and/or gasification.
WO 2005/106327, which is hereby incorporated by reference in its entirety, discloses a cyclonic plasma pyrolysis/vitrification system pyrolyzing and vitrifying waste materials into exhaust gas and slag using a plasma torch. This system reduces toxic materials such as heavy metals. This system melts fly ash after being absorbed at the inner walls of a reactor under the centrifugal force formed by the plasma torch. In this system, the plasma torch is inclined at a predetermined angle with respect to an internal bottom surface of the reactor. This system includes an auxiliary reactor for receiving exhaust gas from the main reactor. This auxiliary reactor is positioned on a side of the main reactor. This system requires an afterburner to increase the temperature of exhaust gases. In addition, this system requires a separator wall exposed to relatively high temperatures on both sides (for example, above about 1400° C.) without a heat sink, thereby risking high temperature failure of this element. This system can also result in erosion of the reactor wall caused by a high power/velocity plasma jet directed between about 20 and 40 degrees above the plane of the surface of impingement.
U.S. Pat. No. 6,910,432, which is hereby incorporated by reference in its entirety, discloses a method for combusting a solid fuel in a slagging cyclone reactor having a burner and a barrel. The method involves injection of two oxidant streams, a first oxidant stream having an oxygen concentration of about 21% by volume and a second oxidant stream having an oxygen concentration greater than the oxygen concentration of the first stream. The two streams are selectively injected into a cyclone combustor whereby mixing of the two oxidant streams is such that a part of the first oxidant stream remains unchanged from its original concentration in the barrel of the combustor. This method does not include a secondary fuel within the cyclonic reactor and can result in erosion of the reactor wall due to high velocity injection.
U.S. Pat. No. 6,968,791, which is hereby incorporated by reference in its entirety, discloses a method for operating a cyclone reactor. The cyclone reactor includes a barrel having a burner end (the front or inlet end) and a throat (the rear or the exhaust end), two burners in communication with the barrel, a stream of primary fuel and primary oxidant, and a stream of secondary fuel and a secondary oxidant, wherein the oxygen concentration of the first oxidant is about 21% by volume and the oxygen of the second concentration is greater than about 21% by volume. The secondary fuel and oxidant are introduced at the burner end. The products of secondary fuel and oxidant combustion exit at the throat end, and the secondary flame generated by the secondary fuel and the oxidant generates a supplemental radiant heat within the cyclone. Additionally, this method can also be prone to refractory erosion.
U.S. Pat. No. 7,621,154, which is hereby incorporated by reference in its entirety, discloses a method for supplying heat to a melting furnace for forming a molten product. A first fuel having an ash component and a first oxidant is introduced into a slagging chamber along with a second fuel and a second oxidant, the second oxidant having an oxygen concentration between about 22% by volume and 100% by volume. At least a portion of the first fuel and a second fuel is combusted within the slagging chamber, while the ash component is collected as a layer of molten slag and is withdrawn from the slagging chamber. Slagging combustor gas effluent is passed from the slagging chamber into a combustion space of the melting furnace at a temperature between about 1000° C. and about 2500° C. to supply heat to form the molten slag.
What is needed is a gasification method and a cyclonic gasifier wherein the temperature and viscosity of slag within the gasifier are maintained, the gasifier is substantially protected from erosion, oxidant(s) use little or no inert gas, gas momentum for gasification is maintained, a compact arrangement provides a high heat release to volume ratio, solid fuel particles can be rapidly heated and/or ignited, and/or residence time and uniformity of temperature distribution can be extended.